Hi, thanks for looking at this.
All the are in the text in the <body> and were inserted in book view by hitting the spacebar.
There are none inside the <head> or <link> tags. But it is the <link> that gets damaged if a is present in the text of that file.
There are four files attached, 2 pure Sigil code and the same two after conversion by Calibre. They all contain exactly the same content but made with 2 different versions of Sigil, as per the filenames.
I did not keep a copy of 0.6.0 so used 0.5.3.
Chapter 1 is an example of a poem with lots of differential spacing. [gives error]
Chapter 2 has one [gives error]
Chapter 3 has only ordinary spaces [no error]
The problem does not exist before the EPUB is opened and it does not appear to matter which version of Sigil made the file originally.
The act of opening EPUBs with Sigil 0.6.1 introduces the problem to files that have been converted by Calibre. The same file can be opened by 0.5.3 with no errors.